In the traditional underwear industry, most underwear is designed with a back buckle. The back buckle is mainly used for allowing a user to adjust the tightness of the underwear. At present, the back buckle on the market basically has such as structure that a plurality of rows of exposed fittings are disposed on a substrate. The number of fittings in each row is the same and varied. The fittings of the existing back buckle are exposed to the outside, and there is no back buckle of underwear with a design of concealed fittings on the market.